2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 208-29
Presentation Time: 4:00 PM

DESCRIPTION OF A GARNET-BEARING GRANOFELS FOUND AT WEST CHICAGO CREEK, COLORADO


LUPO, Nicholas S., Department of Geological Sciences, SUNY Geneseo, 1 College Circle, Geneseo, NY 11454 and FARTHING, Dori J., Department of Geological Sciences, SUNY-Geneseo, 1 College Circle, Geneseo, NY 14454, nsl2@geneseo.edu

A garnet-bearing granofels is found along West Chicago Creek in the Front Range of the Colorado Rocky Mountains. The outcrop is located east of Georgetown and north of Mount Evans and is situated in an area mapped as Precambrian quartz monzonite and granodiorite. Also nearby are Precambrian metamorphic units described as biotitic schists, genisses, and migmatites. The West Chicago Creek granofels is likely similar in age to these units.

Approximately 80% of the West Chicago Creek granofels is composed of quartz and plagioclase. The remaining phases are, in order of abundance, biotite, muscovite, garnet, and accessory zircon. The quartz and plagioclase crystals are anhedral and sub-euhedral respectively. Some quartz crystals also exhibit elongate subgrains due to deformation. Sutured boundaries are found between the two most abundant phases but when the crystals are larger the boundaries are typically straight. The crystals vary in size from 0.04 mm to 5.2 mm. The smaller crystals are concentrated in zones that are interpreted to represent recrystallization. These recrystallization zones do not appear to be aligned within the body of the rock.

The garnet porphyroblasts are heavily fractured. The garnets, which belong to the pyralspite series, are also commonly rimmed by biotite. The relatively low abundance of mica and the absence of fabric makes the West Chicago Creek granofels unique compared to the other metamorphic units in the region.